[phil21]

To make a different argument - because Fedex and UPS cherry-pick the profitable routes and leave USPS to handle the undesirable markets they are required by law to serve. This would be highly problematic if the USPS didn't exist on those routes as a very subsidized backstop.

That's about the only argument you can make, and I'm not certain it's a great one. Additionally one giant national parcel company in theory should have scale working for them and thus lower per unit costs - in practice I think the opposite holds true for large monopolies. But reasonable minds could disagree here.